1. Field of the Invention
The embodiments discussed herein are related to a control circuit of a switching power supply device that includes a current resonance-type DC-DC converter to cope with a wide range of input voltages for worldwide compatibility.
2. Background of the Related Art
In a current resonance-type switching power supply device, control that has a target voltage outputted is performed by a control circuit disposed on the primary side of a transformer based on a signal fed back from the secondary side of the transformer. However, since there is no way to directly detect the load current, the control circuit detects the current on the primary side and estimates the load current on the secondary side. Here, when the estimated load current exceeds a current value set in advance, the current circuit performs an overload protection operation to protect the switching power supply device from an overload state.
Since the input voltage of a switching power supply device with worldwide compatibility is not constant, during an overcurrent protection operation, the control circuit performs control in keeping with the input voltage. That is, even in a state where the load current of the switching power supply device is the same, when the input voltage changes, the control circuit needs to change the threshold for overload protection in keeping with the input voltage. As one example, when the threshold for overload protection for the primary-side current is 2 A for an input voltage of 100V, the threshold for overload protection for the primary-side current is set at 1 A for an input voltage of 200V.
For this reason, the control circuit of a switching power supply device with worldwide compatibility needs information on the input voltage in order to change the threshold for overload protection. A typical way of acquiring information on the input voltage is to divide the input voltage and input a voltage into the control circuit (see, for example, Japanese Laid-open Patent Publication No. 2012-170218 (FIGS. 1, 2, and 5)).
According to Japanese Laid-open Patent Publication No. 2012-170218, the input voltage is divided so that information on the input voltage may be directly acquired, and by applying the acquired information on the input voltage to the detection current on the primary side inputted into an overload protection circuit (“estimated load information”) or to threshold information, overload protection is performed in keeping with the input voltage. This means that with Japanese Laid-open Patent Publication No. 2012-170218, to detect the input voltage, a circuit for dividing the input voltage needs to be provided outside the control circuit. In addition, to input the divided input voltage, there is an increase in the number of package pins of the controller integrated circuit that forms the control circuit, which increases the cost of the controller integrated circuit.
On the other hand, a method of acquiring information on the input voltage without using a means that inputs the input voltage via a divider circuit and package pins of a controller IC is also known (see, for example, International Publication Pamphlet No. WO2011/132275 (FIG. 1)). In WO2011/132275, two switching elements are connected in series to construct a half-bridge circuit and information corresponding to the input voltage is acquired from a midpoint of the switching elements. This configuration makes use of the fact that a voltage which substantially corresponds to the input voltage is outputted at the midpoint of the two switching elements when the high-side switching element is on. The voltage outputted at the midpoint of the two switching elements is supplied, via an overcurrent protection diode and resistors for dividing voltage, to an input terminal that detects an overcurrent on the primary side of the controller integrated circuit. By doing so, since information corresponding to the detection current on the primary side includes information corresponding to the input voltage, correction may be performed while eliminating the dependency on the input voltage.
However, although using the voltage outputted at the midpoint of the two switching elements as information on the input voltage eliminates the need of providing the controller integrated circuit with a new terminal, it is difficult to perform correction accurately. Also, a diode for preventing back flow and resistors for dividing the voltage outside the control circuit are additionally needed.